Copper-based materials have currently supplanted aluminum-based materials as the material of choice for interconnects in integrated circuits (“ICs”). Copper offers a lower electrical resistivity and a higher electromigration resistance than that of aluminum, which has historically been the dominant material used for interconnects.
Interconnects in ICs are becoming one of the dominant factors for determining system performance and power dissipation. For example, the total length of interconnects in many currently available ICs can be twenty miles or more. At such lengths, interconnect resistance-capacitance (“RC”) time delay can exceed a clock cycle and severely impact device performance. Additionally, the interconnect RC time delay also increases as the size of interconnects continues to relentlessly decrease with corresponding decreases in transistor size. Using a lower resistivity material, such as copper, decreases the interconnect RC time delay, which increases the speed of ICs that employ interconnects formed from copper-based materials. Copper also has a thermal conductivity that is about two times aluminum's thermal conductivity and an electromigration resistance that is about ten to about one-hundred times greater than that of aluminum.
Copper-based interconnects have also found utility in other applications besides ICs. For example, solar cells, flat-panel displays, and many other types of electronic devices can benefit from using copper-based interconnects for the same or similar reasons as ICs.
Due to difficulties uniformly depositing and void-free filling trenches and other small features with copper using physical vapor deposition (“PVD”) and chemical vapor deposition (“CVD”), copper interconnects are typically fabricated using a Damascene process. In the Damascene process, a trench is formed in, for example, an interlevel dielectric layer, such as a carbon-doped oxide. The dielectric layer is covered with a barrier layer formed from, for example, tantalum or titanium nitride to prevent copper from diffusing into the silicon substrate and degrading transistor performance. A seed layer is formed on the barrier layer to promote uniform deposition of copper within the trench. The substrate is immersed in an electroplating solution that includes copper. The substrate functions as a cathode of an electrochemical cell in which the electroplating solution functions as an electrolyte, and the copper from the electroplating solution or a consumable anode is electroplated into the trench responsive to a voltage applied between the substrate and an anode. Then, copper deposited on regions of the substrate outside of the trench is removed using chemical-mechanical polishing (“CMP”).
Regardless of the particular electronic device in which copper is used as a conductive structure, it is important that an electroplating process for copper be sufficiently fast to enable processing a large number of substrates and have an acceptable yield. Additionally, the cost of the electroplating solution is also another factor impacting overall fabrication cost of electronic devices using copper. This is particularly important in the fabrication of solar cells, which have to cost-effectively compete with other, potentially more cost-effective, energy generation technologies. Thus, it is desirable that copper electroplating solutions be capable of depositing copper in a uniform manner (i.e., high-throwing power) and at a high-deposition rate.
A number of electroplating solutions are currently available for electroplating copper. For example, sulfate-based electroplating solutions are commonly used for electroplating copper. Some alkaline copper electroplating solutions have a high-throwing power, but are not capable of rapidly depositing copper without compromising the deposited film quality. At high-deposition rates, the copper may grow as dendrites as opposed to a more uniformly deposited film. Additionally, alkali elements (e.g., sodium and potassium) in such alkaline copper electroplating solutions can diffuse into silicon substrates and are deep-level impurities in silicon that can compromise transistor performance. Fluoroborate electroplating solutions can be used for high-speed deposition of copper. However, fluoroborate electroplating solutions can be more expensive than, more traditional, sulfate-based solutions. Moreover, fluoroborate electroplating solutions may be more hazardous and difficult to dispose of than many other electroplating solutions for electroplating copper.